The present invention is directed to devices for determining the state of motion of a platform that is moving under the influence of gravity. It has particular application to the control of spin-stabilized projectiles.
Projectiles that are to be controlled after firing need to include some sort of mechanism to determine changes in the state of motion of the projectile. In the case of projectiles that are spin-stabilized--that is, in the case of projectiles that are spinning about an axis directed roughly in the direction of motion--sensor outputs have to be mathematically "de-spun" in order to allow the motion determination to be made in an inertial reference frame.
One type of motion-sensing mechanism that has been employed in the past uses an array of accelerometers and employs state-estimation techniques to transform the accelerometer outputs into the motion-state variables. In order to transform the accelerometer outputs accurately, the orientation of the spinning projectile with respect to gravity--which can be thought of as the phase of its spin--must be determined. In the past, this determination has typically been made by employing a device such as a sun sensor for determining which way is up.
Clearly, the use of such a sensor introduces additional mechanical complexity and requires that the sun be visible. Furthermore, it makes it necessary for the system to be supplied with latitude, longitude, and time-of-day information.
An object of the present invention is to permit motion sensing of spin-stabilized projectiles and similar platforms without the use of an external reference to determine the spin phase of the platform.